1. Field of the Invention
The present invention relates to a pacer system which is adapted to alter the rate of the pacer pulses delivered (rate of pacing pulses delivered by an artificial pacemaker) to a heart while an individual is exercising relative to, and utilizing, the partial pressure of carbon dioxide, pCO.sub.2, in the blood in a heart to obtain a required cardiac output.
2. Description of the Prior Art
Heretofore patients with heart dysfunctions or heart disease such as sinus node disease have been able to live a relatively normal life with the implantation of an artificial pacemaker often referred to as a pacer. However, such pacers have not always been able to mimic the response of a normal healthy heart. A normal heart responds to exercise and stress by increasing cardiac output through increased heart rate and/or stroke volume.
In this respect, patients with sinus node disease have lost the ability to increase heart rate with exercise. Accordingly, it has become a goal of optimal pacing to provide exercise responsiveness in a pacer by sensing the need for increased cardiac output.
With a view towards obtaining this goal, a number of pacemakers have been proposed for indirectly sensing the need for increased heart rate by sensing P-waves, nerve impulses, Q-T interval, pH, oxygen saturation, respiratory rate, stroke volume, motion, atrial pressure and temperature.
A P-wave triggered artificial pacemaker adapted to be exercise responsive by responding to average atrial rate is proposed in the Knudson & Amundson U.S. Pat. No. 4,313,442.
An artificial pacemaker responsive to changes in the Q-T interval is proposed in the Rickards U.S. Pat. No. 4,228,803.
The Heilman et al. U.S. Pat. No. 4,303,075 discloses a method and apparatus for maximizing stroke volume through AV pacing using an implanted cardioverter/pacer which is programmed with an AV delay tailored to the particular patient. The mechanism detects and processes the impedance of the heart across two electrodes in contact with heart muscle during each heart cycle and uses the changes from cycle to cycle to trigger the issuance of pulses from the pacer depending on the direction of the impedance changes to maximize stroke volume of the heart, which is proportional to the change in value of impedance between the minimum and maximum detected impedance per heart cycle.
The Funke U.S. Pat. No. 4,312,355 discloses a dual pace-dual sense cardiac pacer which is able to stimulate the atrium and/or the ventricle and which is able to entrain the ventricle, when the atrial rate increases, while preventing bradycardic episodes. The pacer action is triggered by detection of naturally occurring atrial and ventricular action or pulses within a predetermined time period.
The Roline U.S. Pat. No. 4,363,325 discloses a multiple mode pacer activated to switch modes relative to heart rate while preventing atrial bradycardia. This is achieved by mode switching of the pacer from an atrial synchronous mode to a ventricular inhibited mode. Such switch of modes is actuated when no atrial activity is sensed within a preset escape interval referred to as a hysteresis period. Reversal of the mode back to the atrial synchronous mode from the ventricular inhibited mode is actuated in response to a detected atrial rate which is higher than a preset, lower, ventricular rate. With this mode switching, the ventricle will not be stimulated twice in quick succession, which overstimulation could cause atrial bradycardia.
A proposal for placing electrodes on the Hering's nerve which extends from receptors in the sinus and glomus carotids is disclosed in the Gonzalez U.S. Pat. No. 4,201,219.
Sensors for sensing blood pH are proposed in the Alcidi U.S. Pat. No. 4,009,721 and the Mauer et al U. S. Pat. No. 4,252,124. Alcidi controls a pacer relative to the level of blood pH sensed.
In the Bornzin U.S. Pat. No. 4,467,807 molecular oxygen is sensed with an oxygen sensor, preferably of the type as disclosed in the Wirtzfeld et al U.S. Pat. Nos. 4,202,339 and 4,299,820. The Wirtzfeld et al patents teach measuring of oxygen saturation of blood using optical techniques. The transmissiveness of light through blood is used by Wirtzfeld et al to measure oxygen concentration. Bornzin teaches using such measurements for controlling the pacing of a heart.
See also the article "A new pacemaker autoregulating the rate of pacing in relation to metabolic needs+ by Cammilli, Alcidi and Papeschi which appeared in "Cardiac Pacing", pages 414-419 Amsterdam-Oxford: Excerpta Medica, 1977 which teaches sensing pH in the right atrium.
Another artificial cardiac pacemaker which increases pacing rate in accordance with an increase in respiration rate is proposed in the Krasner U.S. Pat. No. 3,593,718.
Pacers for sensing motion or mechanical activity are proposed in the Dahl U.S. Pat. No. 4,140,132 and the Anderson et al U.S. Pat. No. 4,428,378.
The Denniston III U.S. Pat. No. 3,815,611 discloses an apparatus which detects muscle contractions through impedance measurement. The device includes an elastomer body whose impedance changes when flexed. The elastomer body is positioned adjacent a muscle such as a heart muscle such that when the muscle contracts, the elastomer body is flexed to provide a change in impedance to a bias voltage supplied thereto. Such electrical signal can be used to control a pulse generator to generate a pulse when a specified period of time has elapsed since the latest heart activity was sensed by the elastomer body.
Heretofore it has been proposed in the Cohen U.S. Pat. No. 3,358,690 to sense pressure in the right atrium and to utilize the pressure sensed to control pacing of an electrode in the right ventricle.
Also, the Zacouto U.S. Pat. No. 3,857,399 discloses, in FIG. 19 thereof, a pressure sensor that measures either left ventricular pressure or intramyocardial pressure. One sensor is located in the left ventricle. Apparently, the pacer coupled to these sensors responds to average base pressure over relatively long periods of time and the pacer system disclosed therein appears to be static and slowly responsive to exercise.
The Sjostrand et al. U.S. Pat. No. 3,650,277 discloses a system for reducing and controlling the blood pressure of a hypertensive patient by electrical stimulation of the carotid sinus nerves, one of the baroreceptor centers of the body. The system incorporates a pressure transducer which is connected to or applied to an artery of a patient and provides electrical signals substantially representing the instantaneous arterial blood pressure of a patient. Upon calculation of a mean arterial pressure, the system is utilized to provide a series of electrical pulses having a predetermined frequency, magnitude and amplitude through an afferent nerve, such as the carotid sinus nerve, to the heart to mimic pulses to the heart occurring naturally in patients having normal blood pressure. These pulses are provided during the first portion of each heart cycle to take over the function of controlling blood pressure that is usually provided by normally functioning baroreceptors in patients who are not hypertensive.
Another artificial cardiac pacemaker which is responsive to exercise by sensing venous blood temperature in the right ventricle of the heart is proposed in the Cook et al U.S. Pat. No. 4,436,092.
As pointed out in the Alcidi U.S. Pat. No. 4,009,721, when an individual is engaging in muscular work or exerting a muscular effort, such as during exercise, the pH, the pO.sub.2 and the pCO.sub.2 of the human blood undergo a modification. More specifically, the pH and the pO.sub.2 decrease and the pCO.sub.2 increases. In view of this fact, Alcidi proposed monitoring the pH of the blood in the right atrium and regulating the rate of stimulating pulses from a pacemaker in relation to the instantaneous variation of the pH of the blood.
In man, however, the pH range of blood is fairly narrow banded as noted in the Maurer et al U.S. Pat. No. 4,252,124. Typically, the pH value for man is between 7.36 and 7.43.
The extreme values for pH in the human organism are typically given as pH 6.8 and pH 7.8. If the pH value deviates from this range serious disturbances can arise which can lead to brain damage and to death. For example, the pH value is lowered (acidosis) due to oxygen deficiency, in the case of kidney failure or a diabetic coma.
On the other hand, the pH value is increased (alkalosis) during exercise and in the case of excess breathing of oxygen in conjunction with anaesthesia or in the case of a lasting acid loss such as occasioned by vomiting.
Thus, measuring the pH changes in blood, although perhaps primarily related to change in pCO.sub.2, is also affected by other factors unrelated to exercise.
As will be described in greater detail hereinafter, the apparatus of the present invention differs from the artificial pacemaker proposed in the Alcidi U.S. Pat. No. 4,009,721 by utilizing an ion sensitive field effect transistor (ISFET) which is mounted on a pacing lead near the tip of the pacing lead and which includes a gas permeable membrane over a solution-filled chamber which is located above the base or gate region of the ISFET.
Further, the method for using the apparatus of the present invention differs from the mode of operation of the Alcidi artificial pacemaker by providing for operation of the apparatus between maximum and minimum pacing/heart rates, relative to the partial pressure of CO.sub.2 (pCO.sub.2) in the blood independent of the pH of the blood.
In the sensing of pCO.sub.2 in a fluid such as blood, one typically measures or senses the pH and relates the pH to pCO.sub.2. In this respect, the following relationship occurs in blood containing oxygen and carbon dioxide: EQU H.sub.2 O+CO.sub.2 .revreaction.H.sup.+ +HCO.sub.3 --
Heretofore, it has been proposed to provide percutaneous carbon dioxide sensors with a CO.sub.2 permeable membrane. In such sensors placed against the skin, CO.sub.2 gas diffusing through the skin passes through the gas permeable membrane and into a chamber within the sensor where a pH sensing electrode is in communication with the liquid in the chamber. As the amount of CO.sub.2 in the liquid increases, the amount of the positive free hydrogen ions increases. These ions collect at the pH sensing electrode causing changes in electrical potential. The changes in electrical potential are then utilized to determine the amount of CO.sub.2 gas in the blood.
Examples of percutaneous CO.sub.2 sensors are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,659,586 Johns et al 4,197,853 Parker 4,401,122 Clark, Jr. ______________________________________
It has also been proposed to provide catheters insertable into a blood vessel with a gas permeable membrane and pH sensor for measuring gases in the blood. Examples of such previously proposed catheters are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,572,315 Cullen II 3,658,053 Fergusson et al 3,710,778 Cornelius ______________________________________
Further, it has been proposed to utilize field effect transistors for measuring chemical properties such as ion activities, including gas ions. Such field effect transistors have taken various forms such as a chemical sensitive field effect transistor (CHEMFET), an ion sensitive field effect transistor (ISFET), an insulated-gate field effect transistor (IGFET), a liquid oxide semiconductor field effect transistor (LOSFET), and a conventional metal oxide semiconductor field effect transistor (MOSFET). Examples of such previously proposed field effect transistors are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 4,020,830 Johnson et al 4,180,771 Guckel 4,198,851 Janata 4,218,298 Shimada et al 4,273,636 Shimada et al 4,411,741 Janata 4,478,222 Koning et al 4,486,290 Cahalan et al ______________________________________
The Janata U.S. Pat. No. 4,411,741 discloses a gap between a bridge member with holes in it over an insulator over a conducting channel or gate region of a CHEMFET.
Also, a solid state reference electrode capable of use with a field effect transistor, such as an ISFET, has been proposed in the Zick et al U.S. Pat. No. 4,450,842 and an AC mode of operating a CHEMFET is disclosed in the Ho U.S. Pat. No. 4,488,556.
Although some of the FETs disclosed in the above patents have a chemically sensitive layer over a gate region of the FET, these patents do not appear to disclose a chamber with a liquid solution therein over a gate region of a FET with a CO.sub.2 permeable membrane closing off the chamber and being adapted to be exposed to blood as provided in the apparatus of the present invention.